Backlights are used to illuminate thick and thin film displays including liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDAs), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays.
LEDs are also advantageous over CCFLs because they require a very short period of time, for example, around one hundred nano-seconds, to switch from full dim to full bright. CCFLs, HCFLs and incandescent lamps can require more than a millisecond to switch from full dim to full bright. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
With the proliferation of inexpensive LCD displays of various sizes, displays are being used in a multitude of applications. For example, LCD displays are now commonly used in automotive applications in devices such as Global Positioning System (GPS) devices and entertainment systems like televisions and DVD players.
To control the intensity of the LED backlight, pulse-width modulation (PWM) is often used. PWM of a signal or power source involves the modulation of its duty cycle, to control the amount of power sent to a load. PWM uses a square wave whose duty cycle is modulated resulting in the variation of the average value of the waveform. PWM alternates between a high voltage that causes the emission of bright light and a low voltage that does not cause the emission of light, instead of providing a continuous voltage to the LED for causing a continuous output of a certain intensity of light.
In PWM, the LED switches quickly enough that the human eye does not perceive the on and off states, but instead perceives an intensity of light that depends on the duration of the on state. Presently, the adjustments to the backlighting are made independently of the images being displayed by the pixel circuitry. For example, a laptop is typically factory set to provide only two different levels of brightness: a higher level of brightness during the full power mode and a lower level of brightness during the battery power mode. Some prior art also discloses adjusting the backlight intensity at the beginning of each frame (see U.S. Pat. No. 7,138,974).
In video production, animation, and related fields, a frame is one of the many still images which compose the complete moving picture. Prior to the development of digital video technology, frames were recorded on a long strip of photographic film, and each image looked rather like a framed picture when examined individually, hence the name. When the moving picture is displayed, each frame is flashed on a screen for a short time (usually 1/24th, 1/25th or 1/30th of a second) and then immediately replaced by the next one. Persistence of vision blends the frames together, producing the illusion of a moving image. The video frame is also sometimes used as a unit of time, being variously 1/24, 1/25 or 1/30 of a second, so that a momentary event might be said to last 6 frames. The frame rate, the rate at which sequential frames are presented, varies according to the video standard in use. In North America and Japan, 30 frames per second is the broadcast standard, with 24 frames per second now common in production for high-definition video. In much of the rest of the world, the rate of 25 frames per second is standard.
This frame-by-frame backlight control of the prior art, in which the backlight is adjusted only once for each frame, has several deficiencies. For example, when a very dark image immediately follows a bright image, the frame-by-frame control technique can result in undesired visual artifacts. Similarly, for the frame in which one portion of the displayed image is bright and another portion is dark, the frame-by-frame control technique can result in undesired visual artifacts. The apparatus and techniques of the present invention overcome these deficiencies and provide other unique features.